If it were my Wingmaster, I'd post a Classifieds ad on this forum (or elsewhere), with a few clear pics of both sides and a detailed description, including any known defects, for sale @ $350, shipping included.
After you get the MO AND the copy of the receiving FFL if the sale is to another state, pack it up & take it to a US Post Office (it's legal for private persons to mail long guns to FFL's in other states), send it Prioriy Mail, insured.
If you're the nervous sort, if it's sent Registered Mail, transport is surer, but will tke longer, since every person who touches the parcel must sign for it. RM is a little more expensive - figure $40 total.
BTW - there is NO obligation to divulge the contents of the parcel, beyond answering the Postal Clerk's questions about it's flammibility, explosiveness, fragility, etc.
I DO have them mark my parcels as "fragile".
Good luck - the 20 ga is popular.